A hydrogen production process based on a two-step thermochemical water-splitting cycle has been widely known before the filing of this patent application. The hydrogen production process is designed to repeat the following two reaction formulas.MOox→MOred+½O2  First StepMOred+H2O→MOox+H2  Second Step
Specifically, this hydrogen production process comprises a first step of reducing a metal oxide MOox to form a reduced metal oxide MOred and produce oxygen through a high-temperature thermal decomposition reaction, and a second step of reacting the reduced metal oxide with water to oxidize the reduced metal oxide to a metal oxide and produce hydrogen.
Typically, magnetite Fe3O4, which is known and described as “iron-based oxide” or “ferrite”, is used as the metal oxide MOox, i.e., a reactive working material for the hydrogen production process. This iron-based oxide as the reactive working material is reduced to wustite FeO in the first step to release oxygen, and the wustite FeO is reacted with water in the second step to release hydrogen and return to magnetite Fe3O4. Then, the reactive working material will be reused.
In the above reaction formulas, the process of releasing oxygen in the first step is generally required to perform under a high-temperature atmosphere of 1800 to 2300° C. In practice, under such a high-temperature atmosphere for the oxygen release reaction, the iron-based oxide is sintered to be deactivated and cause quite strong vaporization. Therefore, it is required to quench the vaporized substance, and this requirement makes it difficult to put the two-step thermochemical water-splitting cycle to practical use.
With a view to solve the problem concerning the reactive working material for use in the hydrogen production process based on the two-step thermochemical water-splitting cycle, the applicant of this patent application previously disclosed a hydrogen production process based on a two-step thermochemical water-splitting cycle which uses a reactive working material comprising ferrite fine powder and zirconia fine powder supporting the ferrite fine powder, in Japanese Patent Application No. 2003-060101.
This reactive working material is formed such that ferrite fine powder is supported on zirconia fine powder. The zirconia fine powder is hardly sintered even at high temperatures, and the ferrite fine particles supported on the zirconia fine powder is prevented from coming in close contact with other ferrite particles each other so as to suppress grain growth thereof to provide enhanced reactivity and reusability even at a relatively low temperature of 1400° C. or less.
Fe3O4 and FeO to be repeatedly formed during the reaction cycle have specific gravities of 5.2 and 5.7, respectively. Thus, due to volumetric changes of these powders during the reaction cycle, the ferrite fine powder will scale off from the zirconia fine powder to spoil the zirconia powder's effect of suppressing grain growth in the ferrite fine powder. Moreover, under a reaction atmosphere repeatedly having a temperature of 1400° C. or more which is greater than a melting point of FeO, the ferrite fine powder will be gradually agglomerated to cause grain growth while repeating melting and solidification, resulting in deterioration of reaction efficiency.